Integrated semiconductor circuit and method for producing it, and use of such a circuit for providing a flow meter

ABSTRACT

An integrated semiconductor circuit including at least two mutually separated wafer parts (3, 4) of semiconductor material and conductors (7) for establishing electrical conection between the wafer parts is disclosed with a thermally insulating jointing substance (9) which is applied across the gap between the wafer parts (3, 4) for holding these parts together. The jointing substance (9) is mechanically supporting for holding the wafer parts (3, 4) together, and the conductors (7) are so dimensioned that they have a negligible supporting function as compared with the jointing substance (9). A method for producing such a cirucit and the use of the circuit for providing a flow meter for measuring the flow velocity of a flowing gaseous or liquid medium are also disclosed.

BACKGROUND OF THE INVENTION

The present invention generally relates to an integrated semiconductorcircuit and a method of producing it, said circuit comprising at leasttwo mutually separated wafer parts of semiconductor material, andconductors for establishing electrical connection between the waferparts which are substantially thermally insulated from each other.

The invention further relates to the use of such an integratedsemiconductor circuit for providing a flow meter for measuring the flowvelocity of a flowing gaseous or liquid medium, said flow meter beingparticularly characterized in that it comprises two mutually separatedwafer parts one of which is adapted to be heated and disposed in theflow and the other of which is also adapted to be disposed in the flow,however without being heated, the flow velocity of the medium beingcalculated on the basis of the dissipation by thermal convection of thefirst wafer part to the ambient flowing medium.

Since the invention is especially well suited for providing flow metersor flow sensors of the above-indicated type, the following descriptionwill be directed to the use of the integrated semiconductor circuit andthe method of producing it especially for such sensors, but it isevident to anyone skilled in the art that this is not the only field ofapplication of the invention which may also be used in all integratedcircuits where it is desirable to achieve thermal insulation betweendifferent parts of a semiconductor circuit and electrical connectionbetween the parts.

In semiconductor technology, a large number of integrated transducers orsensors of semiconductor material have been developed lately, such astemperature and flow sensors, which are manufactured along the sameprinciples as conventional integrated circuits, i.e. are built up on alayer of monocrystalline semiconductor material on which the electriccomponents and conductors required for the operation of the sensor havebeen integrated according to known techniques. In addition to the smallsize of the sensor and the reduced costs of manufacture of the sensorper se, achieved by producing several identical units at a time (batchprocess), a further advantage is gained, namely that the signalprocessing electronic circuits or equivalent components associated withthe sensor can be directly integrated on the sensor in connection withthe manufacture thereof, which further reduces the cost of the sensorand enhances its reliability. The measuring performance of the entiresystem is also improved by directly integrating the signal processingelectronic circuits on the sensor. A first signal gain can then beachieved closer to the measuring unit proper, which prevents weaksignals from disadvantageously being fed over long signal paths.

A known flow velocity sensor of this type comprises a thin, narrowsilicon beam, a base plate fixedly connected to one end of the siliconbeam and carrying bonding pads required for the operation of the sensor,and a sensor part, also of silicon, fixedly connected to the other endof the silicon beam. For using the sensor, said other end of the beam isinserted through a tube wall or the like into a flow of gas or liquid,the velocity of which should be measured, so as to place the sensor partin the flow. The mode of operation of the sensor, which is based onknown techniques, is as follows. The sensor part is electrically heatedby means of a resistor integrated thereon, to an upper temperature,whereupon the sensor part is allowed to cool to a lower temperature as aresult of the dissipation by thermal convection, it being possible torepeat this process cyclically. Both the heating time and the coolingtime are indicative of the flow velocity of the medium. A firsttemperature-sensitive diode for compensating for temperature variationsin the medium is integrated in the silicon beam, and a secondtemperature-sensitive diode is integrated in the sensor part, it beingpossible by means of this second diode and the resistor to provide atemperature feedback control system for controlling the temperature ofthe sensor part.

In order to achieve accurate flow measurements with the flow sensordescribed above, it is obviously desirable that the sensor part bethermally insulated from the silicon beam, such that the temperature ofthe sensor part is substantially affected by the dissipation by thermalconvection because of the flow and not because of thermal conductionbetween the sensor part and the beam.

To this end, the circuit in the above-mentioned previously known flowsensor has been formed into two physically separated units or waferparts which are held together only by the conductors extending betweenthe wafer parts (sensor part and beam) and also providing an electricalconnection between the wafer parts. This solution however suffers from aserious drawback. So that the conductors should have a sufficientsupporting capacity, i.e. in order that the beam in the above-describedflow sensor should be able to support the sensor part, they must have arelatively large thickness, which means a large conductorcross-sectional area in the joint between the sensor part and the beamand, thus, entails undesired thermal conduction by the conductors fromthe sensor part to the beam, which in turn adversely affects thesensitivity and speed of the sensor device. If the conductors are madethinner to prevent such undesired thermal transfer between the sensorpart and the beam, the sensor device will become more vulnerable toimpacts and more easily damaged. There is also a risk that a sensordevice of the above-defined type will be damaged because of the pressurefrom the ambient flowing medium.

SUMMARY OF THE INVENTION

In order to solve the problem outlined above, the present invention hasprovided an integrated semiconductor circuit of the type mentioned inthe introduction to the specification, in which a thermally insulatingjointing substance is applied across the gap between the wafer parts forholding these together. The jointing substance then preferably ismechanically supporting for providing a strong bond between the waferparts, and the conductors are preferably so dimensioned that they have anegligible supporting function as compared with the jointing substance.Thus, this construction will overcome the above-mentioned problem andachieve the above-mentioned object, i.e. provide thermal insulationbetween the wafer parts, electrical connection therebetween andmechanical interconnection thereof.

In a preferred embodiment of the semiconductor circuit according to theinvention, the jointing substance is so applied in the gap between theopposing narrow sides of the wafer parts that the wafer parts and thejointing substance form a unit of substantially

uniform thickness, and the conductors consist of flat metal conductorsextending between the wafer parts and, optionally by an oxide applied tothe conductors, directly engaging the jointing substance.

In addition to the above-mentioned supporting function, the jointingsubstance may also have a protective function. To this end, the jointingsubstance, apart from being applied in the gap, is also applied as athin protective layer over the conductors and, optionally, on part ofone flat side of the wafer parts.

The jointing substance, which may be any thermally insulating andpreferably mechanically supporting material, preferably consists of anorganic material, such as polyimide, which is a most heat resistant andmechanically strong material.

When using the integrated semiconductor circuit according to theinvention for making a flow meter of the type described above, the beamand the sensor part are held together by means of said thermallyinsulating jointing substance such that the sensor part is supported bythe beam by the intermediary of the jointing substance. The sensor partis electrically heated through said conductors, the totalcross-sectional area of which in the gap between the sensor part and thebeam is limited for restricting losses by thermal conduction of thesensor part to the beam, via the conductors.

In order to produce the integrated semiconductor circuit describedabove, the present invention provides a method therefor, in which thecircuit is first integrated with optional components and said conductorsapplied in a desired pattern on the front side of the circuit. Themethod of the invention is characterized by the steps of providing alayer applied to the backside of the circuit, removing the semiconductormaterial in the gap or gaps between the desired wafer parts, wherebythese parts are held together substantially by means of said backsidelayer and whereby the conductors form bridges over the gap or gapsbetween the wafer parts, and applying the thermally insulating jointingsubstance over the gap or gaps.

By using the backside layer, which may consist of a silicon dioxide or ametal, two advantages are gained. First, the layer serves to hold thewafer parts together in that step of manufacture where the semiconductormaterial in the gap or gaps has been removed and the jointing substancehas not yet been applied in the gap. Secondly, in the case where thejointing substance is applied to the circuit from above, the backsidelayer prevents the jointing substance from contacting the backside ofthe circuit. After the application of the jointing substance, thebackside layer can be removed, whereupon the jointing substance alonewill hold the respective wafer parts together.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail hereinbelow withreference to a particularly preferred embodiment of an integratedmulti-part flow meter which is based on the integrated semiconductorcircuit according to the invention, and to a preferred method ofproducing the semiconductor circuit according to the invention.

In the accompanying drawings, to which reference is now made,

FIG. 1 is a perspective view of a gas flow meter of a known design, inwhich the different semiconductor wafer parts are only held together bymeans of conductors.

FIG. 2 is a schematic side view of a joint corresponding to the jointbetween the wafer parts in the flow meter of FIG. 1 but where the waferparts are instead joined together by using the semiconductor circuit andthe method according to the invention, and

FIGS. 3A-3E schematically illustrate the method of the invention forproducing the flow meter described in connection with FIGS. 1 and 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The known gas flow sensor or meter as shown perspective in FIG. 1 ismade up of three main parts, namely a base plate 1 having fiveelectrical bonding pads 2 which are applied to the front side of thebase plate and by means of which the sensor can be connected to externalcircuits and drive means, a silicon beam 3 extending from the base plate1 and having a thickness in the order of 30 μm, and a sensor part 4 inthe form of a small silicon chip disposed at the end of the silicon beam3 facing away from the base plate 1. The beam 3 and the sensor chip 4are inserted through an opening 5 in a tube wall 6 or the like definingthe flow of the gas, the velocity of which should be measured by theflow sensor. As shown in the Figure, the sensor chip 4 is arranged withits flat side parallel to the direction of flow indicated by the arrowA. A resistor R and a first diode D1 are integrated on the front side ofthe sensor chip 4, and a second diode D2 is integrated on the front sideof the silicon beam 3. These three components R, D1 and D2 areelectrically connected to the bonding pads 2 by four flat metalconductors 7.

The mode of operation, which is based on a known technique, of theillustrated sensor during gas flow measurements will now be described ingreater detail. The sensor chip 4 is heated by means of the resistor Rto an upper temperature T₁ whereupon the chip as a result of dissipationby thermal convection to the ambient flowing gas will cool to a lowertemperature T₂. This heating and cooling process can be cyclicallyrepeated. In the measuring process, use is made of the fact that a p-njunction in silicon, i.e. diodes D1 and D2, changes its forward voltagedrop by about -2 mV/°C. at constant current in the forward direction ofthe diodes. The first diode D1, which is applied to the sensor chip 4,forms together with the resistor R a temperature feedback control systemfor controlling the temperature of the sensor chip 4 in said cycle. Theother diode D2 disposed on the silicon beam 3 is used for compensatingfor variations in the gas temperature. By measuring the dissipation bythermal convection from the sensor chip 4 on the basis of the differenttemperature and power consumption processes of the sensor chip 4 duringthe heating and cooling cycle, it is thus possible to obtain a measuringvalue of the flow velocity of the gas.

In order to achieve a high accuracy and/or sensitivity and speed of thesensor, it is obviously desirable to provide thermal insulation betweenthe beam 3 and the sensor chip 4. In the gas flow meter of known designshown in FIG. 1, attempts have been made to solve this problem bydesigning the beam 3 and the sensor chip 4 as two physically separatedunits. The above-mentioned metal conductors 7 are then also used formechanically holding together the two parts at the gap 8. In order toobtain a sufficient supporting capacity of the conductors, these havebeen reinforced by electroplating to ensure that the final thickness ofthe conductors has been in the order of 10 μm or more. Such aconsiderable thickness of the conductors has resulted in that thedissipation by thermal convection from the sensor chip 4 to the beam 3via the conductors 7 has had a relatively great influence when measuringand calculating the gas flow velocity, this reducing the accuracy of thesensor. As a result of the larger mass of the conductors 7, the speed ofthe sensor is also reduced.

In FIG. 2, which is a side view on a larger scale of the sensor chip 4and the beam 3, the above-mentioned problem inherent in the flow sensordescribed above has been solved by using the integrated semiconductorcircuit and the method according to the present invention. In theillustrated embodiment of the invention, the beam 3 and the sensor chip4 are still designed as two physically separated units, but themechanical joining of the two parts now is not accomplished by theconductors 7, which merely serve to establish electrical connectionbetween the beam 3 and the chip 4, but instead by means of a thermallyinsulating and mechanically supporting jointing substance 9 which isapplied in the gap between the opposing narrow sides 10 of the beam 3and the chip 4 in such a manner that the beam 3, the jointing substance9 and the chip 4 form a unit of substantially uniform thickness. Thethickness of the conductors 7, which now need not have any supportingfunction, has been reduced to about 1 μm or less in the embodimentillustrated in FIG. 2. The use of the semiconductor circuit according tothe invention and the method for producing it thus provides a strongmechanical supporting connection as well as an electrical connection,without any undesired heat transfer over the gap.

The method according to the invention for producing the integratedsemiconductor circuit will now be described in more detail withreference to FIGS. 3A-3E illustrating different steps in the manufactureof the flow sensor described in connection with FIGS. 1 and 2.

According to a known technique, the monocrystalline silicon wafer isprocessed, for instance by means of a crystal orientation-dependentsilicon etch, into the shape illustrated in FIG. 3A. In this step, it isassumed that the conductors 7, the resistor R and the diodes D1 and D2are integrated on the top face of the silicon wafer.

In a first step (FIG. 3A) according to the invention, there is provideda backside layer 11 consisting of a silica layer about 1 μm thick on thebackside of the circuit.

In a second step (FIG. 3B), the semiconductor material is removed byetching at the location where the joint 8 between the silicon beam 3 andthe sensor chip 4 should be formed and at the edges around these twoparts, the beam 3 being fixed at its end facing away from the sensorchip 4 to a supporting or protective frame 14 (see FIG. 3E). By exposingthe gap, the beam 3 and the sensor chip 4 will be held togethersubstantially by means of the backside layer 11, whereby the conductors7 extending between the beam 3 and the sensor chip 4 form bridges overthe gap. The backside layer 11 is then "stretched" between the chip 4,the beam 3 and said protective frame 14.

In a third step (FIG. 3C) according to the invention, the thermallyinsulating and mechanically supporting jointing substance 9 is appliedin the gap from above. The jointing substance 9 preferably consists ofan organic material, such as polyimide, which after thermosettingprovides a strong bond between the sensor chip 4 and the beam 3. Thebackside layer 11 then prevents the jointing substance 9 frompenetrating down onto the underside of the circuit. As shown in FIG. 3C,there is also applied in connection with the application of thepolyimide, a thin layer of the same jointing substance over the sensorchip 4, the conductors 7 and the beam 3 and, in practice, this thinlayer is formed over the entire semiconductor wafer which should laterbe broken up into separate sensors. Part of this layer can be maintainedafter completed manufacture in order to provide a protective layer forthe flow sensor. At any rate, it is preferable that the jointingsubstance, at least at the location of the joint, projects slightly overthe chip 4 and the beam 3 to ensure a strong interconnection of theseparts. (See FIG. 3D).

In a final, fourth step (FIG. 3D) according to the invention, thebackside layer 11 can be removed from the circuit or flow sensor,whereby the sensor chip 4 is supplied by the silicon beam 3 by means ofthe jointing substance 9 alone.

FIG. 3E is a top plan view of the finished flow sensor which, inaddition to the above-mentioned parts, also includes a protective frame12 which is fixed along fracture lines 13 to the base plate 1 at adistance from the silicon beam 3. This protective frame is intended tobe broken apart at said fracture lines prior to using the flow sensor.

The invention must of course not be considered restricted to theembodiment described above and illustrated in the drawings, but may bemodified in various ways within the spirit and scope of the patentprotection as claimed. For instance, the flow meter may thus also be ofa type in which the temperature of the sensor chip is maintainedconstant and the current flow through the heating resistor R thus variesin dependence upon the prevailing flow velocity. In this case, themeasuring value is calculated on the basis of power consumption.Further, the semiconductor material may be other than silicon, e.g.GaAs, or combinations of different semiconductor materials. As analternative, the jointing substance may be applied only over the topface of the wafer parts as a layer for holding the wafer parts together,such that no jointing substance is applied in the gap between the narrowsides of the wafer parts facing each other.

I claim:
 1. A flow meter for measuring the flow velocity of a flowinggaseous or liquid medium, said flow meter comprising an integratedsemiconductor circuit having a first wafer part (4) with a plurality ofthin conductors thereon and adapted to be heated and disposed in theflowing medium, a second wafer part (3) with a plurality of thinelectrical conductors thereon and adapted to be disposed in the flowingmedium without being heated, a thermally insulating jointing substance(9) joining the waver parts (3, 4) for holding said first waver part (4)and said second wafer part (3) together such that the first wafer part(4) is supported by the second wafer part (3) by way of the jointingsubstance, and a plurality of thin electrical conductors (7) connectingelectrical conductors on said first wafer part with electricalconductors on said second wafer part and having a limitedcross-sectional area in the gap (8) to restrict thermal conduction bythe conductors (7) from the first wafer part (4) to the second waferpart (3).
 2. A flow meter as claimed in claim 1, wherein said jointingsubstance (9) is mechanically supporting for holding said wafer parts(3, 4) together and that said conductors (7) are so dimensioned thatthey have a negligible supporting function as compared with saidjointing substance (9).
 3. A flow meter as claimed in claim 1, whereinsaid jointing substance (9) is so applied in the gap (8) between thenarrow sides (10) of said wafer parts which are facing each other thatsaid wafer parts (3, 4) and said jointing substance (9) form a unit ofsubstantially uniform thickness.
 4. A flow meter as claimed in claim 1,in which said conductors (7) consist of flat metal conductors extendingbetween said wafer parts, and in which said conductors directly engagesaid jointing substance (9).
 5. A flow meter as claimed in claim 3,wherein said jointing substance (9), apart from being applied over saidgap, is applied as a thin protective layer over said conductors (7) andon part of one flat side of said wafer parts (3, 4).
 6. A flow meter asclaimed in claim 1, wherein said jointing substance (9) is an organicmaterial, such as polyimide.
 7. A flow meter as claimed in claim 1,wherein at least one of said wafer parts in the circuit has means (R)for heating said wafer part (4), said means (R) being driven via saidconductors (7) connected to said wafer part (3).
 8. A flow meter asclaimed in claim 1, wherein at least one of said wafer parts in thecircuit has means (D1, D2) for measuring the temperature of said waferpart.
 9. A flow meter for measuring the flow velocity of a flowinggaseous or liquid medium, said flow meter comprising an integratedsemiconductor circuit having a first wafer part (4) having a pluralityof thin conductors thereon and adapted to be heated and disposed in theflowing medium, a second wafer part (3) having a plurality of thinelectrical conductors thereon and adapted to be disposed in the flowingmedium without being heated, a thermally insulating jointing substance(9) in the gap (8) between the wafer parts (3, 4) for holding said firstwafer part (4) and said second wafer part (3) together such that thefirst wafer part (4) is supported by the second wafer part (3) and thejointing substance, and a plurality of thin electrical conductors (7)connecting electrical conductors on said first wafer part withelectrical conductors on said second wafer part and having a totalcross-sectional area in the gap (8) substantially equal to thecross-sectional areas of the connected electrical conductors to restrictthermal conduction by the conductors (7) from the first wafer part (4)to the second wafer part (3).